Pretreatment Problem Solver
Brad Gruss, Pretreatment & Process
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Problem: C.B. of Phoenix tells me he's having powder adhesion problems with die-cast aluminum parts. He says the titration, nozzles, reverse-osmosis water, dry-off oven, temperature, and time are functioning correctly. He says some parts exhibit both shiny and dull areas. He says the powder seems to adhere well on the shiny areas. He says his company can't believe they're the only supplier and vendor that's had problems with these parts.
Solution: Don't let anyone ever tell you you're the only one having problems with a particular part. This usually happens with a part that gets bounced around from job shop to job shop. The excuse is usually based on price, but the real issues are typically poor process control during casting, postmachining, tumbling or mass finishing, and storing or post-corrosion-inhibiting steps. Simply put, you either have problems with the part before you receive it or problems with the part in your process. Let's take a look at these issues.
Casting. Generally, casting-related problems occur with die-release or the application of the die-release. Usually you'll see fish eyes or craters in the powder caused by insufficient initial cleaning or postcleaning soil bleed-out. The dull or shiny substrate appearance may or may not be related to the casting process. Sometimes you can have areas on a casting that are dull or shiny because of how the molten metal was added to the mold. In addition, with multidimensional castings, you may find areas that cool rapidly or slowly. This can create what is referred to as cold or hot spots, which are areas on the casting that are harder or softer in regards to chemical treatment. A postchemical treatment with a cleaner or phosphate can't differentiate one spot from another; consequently, you receive inconsistent chemical micro-etching.
Post machining. Adhesion problems occur occasionally near areas that have been milled or machined. These problems, like casting problems, stem from poor cleaning or rinsing, poor powdered edge coverage, insufficient cure, or casting porosity.
Tumbling and mass finishing. Postcasting processes, which include a deburring, tumbling, or mass-finishing step, can either create powder adhesion problems or improve substrate adhesion. Here are some questions you should ask about this type of operation:
- What is the process?
- What type of media is used in the operation?
- How consistent is the process control so that each part is treated the same?
- How consistent is the water and chemical flow in terms of gallons per minute (GPM)?
- How are parts rinsed (with a post-rust-inhibiting rinse)?
- How are parts dried (in a basket on the part or through some form of oven)?
Inconsistency of part appearance (some shiny, some dull) may mean problems stemming from the questions listed above. Part history is an area you should thoroughly investigate. You should be receiving these parts either dull or shiny but not both ways on the same part if there's a post-mass-finishing operation after casting.
Storage and corrosion. If these parts are stored in inventory, or if they receive some form of post-rust-inhibiting step, you may find that the storage conditions create oxidation or that the post-chemical-inhibiting step is inadequate. This could create a nonuniform substrate as well. You should consider investigating this area.
Checking problems in your process
C.B. tells me his pretreatment temperatures, titrations, and other variables are up to speed with current specifications. But he doesn't tell me specifics about the pretreatment process, including how many stages there are or what chemicals he uses. This may be important information. Generally speaking, the four minimum requirements for nonferrous preparation are
- 100 percent cleaning, or the removal of all organic soils
- A micro-etching or deoxidation of the aluminum
- A conversion coating (optional in some cases)
- A high-quality postrinse of deionized or reverse-osmosis (RO) water
I have multiple questions for C.B. One question is, are the parts water-break-free
after chemical treatment and RO rinsing? Do the parts go into the pretreatment
process looking one way and coming out either dull or shiny? This would
indicate that the problems are in the washer. If the raw parts were virtually
unchanged in appearance after washing, this would also indicate potential
washer or chemical selection problems. My guess is that you have a typical
five-stage system with quality chemicals from a recognized source. The
real problem for many finishers is that the raw parts or castings are
a rolling target. In effect, you develop your pretreatment process chemistries
for one aluminum casting condition, but the actual parts should be considered
as two or three different substrate conditions. What I'm saying is that
if the castings are shiny, they may need more etch or deoxidation. If
the parts are dull or oxidized, they may need to be further etched either
chemically or mechanically to accept good adhesion. If the parts are
very dull and have a gray metallic smut, they may have been overetched,
causing a soft skin condition that results in adhesion loss.
What I've tried to lay out here are some of the various problems you may find in the pretreatment of castings. I've also given you the opportunity to investigate the cause of the intermittent adhesion problem. You may consider bringing in a technical person from your chemical vendor to try various potential chemical adjustments to the existing chemistry or potentially new chemistries. You should also track the part and its history all the way back to the caster and determine if any subsequent operations or processes come in contact with the part. Finally, recognize the time frame and storage conditions these parts go through before entering your shop. With this information and some testing on your part, the solution should be obtainable. PC
Brad B. Gruss is president of the consulting firm, Pretreatment & Process Inc., RR1 Box 118A, Ashby, MN 56309; 218/747-2320. He specializes in training, troubleshooting, and independent line audits for pretreatment processes. In his more than 25 years in the industry, he has made numerous presentations on pretreatment for powder coating. He holds a BS in business administration and marketing and a BFA in art advertising from Mankato State University, Mankato, Minn. He is a member of the Powder Coating Institute, Chemical Coaters Association, and Association for Finishing Processes of the Society of Manufacturing Engineers.